(1) Field of the Invention
The present invention relates to reverse cycle heat pump and more particularly to a control system for providing defrosting and supplemental heat in the heating cycle.
(2) Description of the Prior Art
Heat pump air conditioners are required to heat an enclosure under all outdoor ambient temperatures. In the heating mode the outdoor heat exchanger functions as the evaporator and accordingly may operate at such low ambient temperatures to cause the accumulation of a coating or layer of frost. Since such a layer of frost operates as a barrier to heat transfer between the evaporator and the air being circulated over the evaporator, the efficiency of the unit is markedly reduced. Also, unless means are provided for interrupting this accumulation of frost, the evaporator can become completely filled or covered with a layer of frost which may eventually cause compressor motor or other damage to the unit.
It has been customary in some present units to shut down the unit until the frost has melted. Another practice is to reverse the refrigerant system so that the outside heat exchanger functions as the condenser to melt accumulated frost. In both of these systems, the heating function is interrupted until the frost is eliminated and the system once again can function in the heat mode. Since these interruptions during extreme cold outdoor ambient temperatures can be frequent, it results in uneven temperatures generally below the accepted comfort range.
U.S. Pat. No. 3,159,981-Huskey, assigned to the General Electric Company, assignee of the present invention, discloses a self-contained air conditioning unit including a reversible refrigeration system and a control circuitry designed to interrupt the operation of the refrigeration system whenever either the outdoor or indoor heat exchanger attains a frosting temperature and to supply auxiliary heat to an enclosure whenever the operation of the refrigeration system is thus interrupted during a heating cycle. The frost control switch includes a vapor-filled bellows and capillary tube sensing element connected to the bellows. The tube is arranged with a first portion in contact with one of the heat exchangers and a second portion in contact with the other heat exchanger whereby the bellows operated switch will stop the compressor when either heat exchanger attains a frosting temperature.
Another prior art attempt at solving the frosting of the outdoor heat exchanger when in the heating cycle it is operating as an evaporator, is disclosed in U.S. Pat. No. 3,466,888-Kyle. The control circuitry includes a first thermistor in heat exchange contact with the outdoor coil and a second thermistor is exposed to the temperature of the outdoor air. The two thermistors are connected in series. When frost forms on the surface of the outdoor heat exchanger, the temperature of the refrigerant therein decreases, and the voltage at the junction of the thermistor changes, and operates a control circuit which stops the fan of the outdoor coil, and reverses the flow of refrigerant so that the outdoor coil operates as a condenser coil to melt the frost.
U.S. Pat. No. 3,348,607-Cootey discloses a split-bulb or dual bulb thermostat wherein a sensing element is located in the path of return air and the other in the outdoor or ambient air. The sensing elements are proportioned in size relative to each other and both elements transmit motion to a single power element in response to changes in the temperature of the air affecting the elements.